This invention relates generally to electrohydraulic directional multi-way control valve systems, and more particularly it relates to a system of the type which includes a preliminary directional control valve and a main directional control valve, the main control valve having a first housing defining an intake port, a return port, and at least two load ports, a first control spool movable in the housing from a central neutral position into one of two lateral working positions, a pressure chamber arranged at each end of the first spool, first spring arranged in each pressure chamber to urge the first spool into its neutral position, control channels connecting, respectively, the pressure chambers to the preliminary control valve, the preliminary control valve having a second housing provided with two control chambers, a second control spool movable in the second housing from a neutral central position into one of two lateral working positions in which pressure fluid is admitted through one of said control chambers into one of said pressure chambers, whereas the other pressure chamber is released through the return port.
A directional control valve system of this kind is known, for example from the publication "Zeitschrift Olhydraulik und Pneumatik," volume 22, 1978, No. 3, page 116. In this known system, the control spool of the preliminary control valve is acted upon by two proportional solenoids arranged at the opposite end faces of the spool and the latter is centered by two springs. If this preliminary directional control valve is employed for the preliminary control of a 4/3-way control valve acting as the main control stage, so in the case of an interruption of the flow of the pressure fluid or in the case of a failure of the controlling electronic the control slider of the main control stage becomes centered in its neutral central position in which a working cylinder connected to the main control stage becomes hydraulically blocked and consequently a malfunction of a controlled machine is prevented. The disadvantage of this prior-art solution resides in the fact that the operational security can be attained only by employing two proportional operating solenoids cooperating with adjustable springs so that the construction expenses are relatively high.